The field of the invention is that of SOI integrated circuits having DRAM arrays of trench capacitor cells.
Dynamic Random Access Memory (DRAM) cells are well known. A DRAM cell is essentially a capacitor for storing charge and a pass transistor (also called a pass gate or access transistor) for transferring charge to and from the capacitor. Data (1 bit) stored in the cell is determined by the absence or presence of charge on the storage capacitor. Because cell size determines chip density, size and cost, reducing cell area is one of the DRAM designer's primary goals. Reducing cell area is done, normally, by reducing feature size to shrink the cell.
Besides shrinking the cell features, the most effective way to reduce cell area is to reduce the largest feature in the cell, typically, the area of the storage capacitor. Unfortunately, shrinking the capacitor plate area reduces capacitance and, consequently, reduces stored charge. Reduced charge means that what charge is stored in the DRAM is more susceptible to noise, soft errors, leakage and other well known DRAM problems. Consequently, another primary goal for DRAM cell designers is to maintain storage capacitance while reducing cell area.
One way to accomplish this density goal without sacrificing storage capacitance is to use trench capacitors in the cells. Typically, trench capacitors are formed by etching long deep trenches in a silicon wafer and, then, placing each capacitor on its side in the trench, orienting the capacitors vertically with respect to the chip's surface. Thus, the surface area required for the storage capacitor is dramatically reduced without sacrificing capacitance, and correspondingly, storable charge.
However, since using a trench capacitor eliminates much of the cell surface area, i.e., that portion of cell area which was formerly required for the storage capacitor, the cell's access transistor has become the dominant cell feature determining array area. As a result, to further reduce cell and array area, efforts have been made to reduce access transistor area, which include making a vertical access transistor in the capacitor trench. See, for example, U.S. Pat. No. 6,426,252 entitled “Silicon-On-Insulator Vertical Array DRAM Cell With Self-Aligned Buried Strap” and references cited in it.
Performance is equally as important as density to DRAM design. Silicon(SOI) has been used to decrease parasitic capacitance and hence to improve integrated circuit chip performance. SOI reduces parasitic capacitance within the integrated circuit to reduce individual circuit loads, thereby improving circuit and chip performance. However, reducing parasitic capacitance is at odds with increasing or maintaining cell storage capacitance. Accordingly, SOI is seldom used for DRAM manufacture. One attempt at using SOI for DRAMS is taught in the cited patent.
In the case of transistors formed on SOI those skilled in the art conventionally introduce a contact to the body to drain away holes generated in the course of operation. Such body contacts add to the area of the cell and thus defeat some of the advantages of using SOI.
Thus, there is a need for increasing the number of stored data bits per chip of Dynamic Random Access Memory (DRAM) products. There is also a need for improving DRAM electrical performance without impairing cell charge storage.